1. Field of the Invention
This invention relates generally to fuel cell systems, and particularly, to the maintenance of adequate hydration in the fuel cell system.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel depending upon the materials that are chosen for the components of the cell and the intended application for which the fuel cell will provide electric power.
Fuel cell systems may be divided into “reformer based” systems (which make up the majority of currently available fuel cells), in which fuel is processed to improve fuel cell system performance before it is introduced into the fuel cell, and “direct oxidation” systems in which the fuel is fed directly into the fuel cell without internal processing.
Because of their ability to provide sustained electrical energy, fuel cells have increasingly been considered as a power source for smaller devices including consumer electronics such as portable computers and mobile phones. Accordingly, designs for both reformer based and direct oxidation fuel cells have been investigated for use in portable electronic devices. Reformer based systems are not generally considered a viable power source for small devices due to size and technical complexity of present fuel reformers. Thus, significant research has focused on designing direct oxidation fuel cell systems for small applications, and in particular, direct systems using carbonaceous fuels including but not limited to methanol, ethanol, and aqueous solutions thereof. One example of a direct oxidation fuel cell system is a direct methanol fuel cell system. A direct methanol fuel cell power system is advantageous for providing power for smaller applications since methanol has a high energy content, thus providing a compact means of storing energy; it can be stored and handled with relative ease; and the reactions necessary to generate electricity can occur under ambient conditions.
DMFC power systems are also particularly advantageous since they are environmentally friendly. The chemical reaction in a DMFC power system yields only carbon dioxide and water as by products (in addition to the electricity produced). Moreover, a constant supply of methanol and oxygen (preferably from ambient air) can continuously generate electrical energy to maintain a continuous, specific power output. Thus, mobile phones, portable computers, and other portable electronic devices can be powered for extended periods of time while substantially reducing or eliminating at least some of the environmental hazards and costs associated with recycling and disposal of alkaline, NiMH and Li-Ion batteries.
The electrochemical reaction in a DMFC power system is a conversion of methanol and oxygen, in the presence of water, to CO2 and water. More specifically, in a DMFC, methanol is introduced to the anode face of a protonically conductive, electronically non-conductive material in the presence of a catalyst and water. This typically includes the use of one or more diffusion layers to manage the flow of reactants and products within the fuel cell. When the fuel contacts the catalyst, hydrogen atoms from the fuel are separated from the other components of the fuel molecule. Upon closing of a circuit connecting a flow field plate at the anode to a flow field plate at the cathode, through an external electrical load, the protons and electrons from the hydrogen atoms are separated, resulting in the protons passing through the material electrolyte and the electrons traveling through a load in the external circuit. The protons and electrons then combine at the cathode with local oxygen, producing water. At the anode, the carbon component of the fuel is converted by combination with oxygen, in the presence of water, into CO2, generating additional protons and electrons. As used herein, “membrane” or “electrolyte” may be used to refer to the protonically conducting, electronically non-conducting material.
The specific electrochemical processes in a DMFC are:Anode Reaction: CH3OH+H2O=CO2+6H++6e Cathode Reaction: 3/2O2+6H++6e=2H2ONet Reaction: CH3OH+ 3/2O2=CO2+H2O
Most commonly in prior art systems, the methanol in a DMFC is preferably used in an aqueous solution to reduce the effect of “methanol crossover.” Methanol crossover is a phenomenon whereby methanol molecules pass from the anode side of the fuel cell, through the electrolyte material, to the cathode side of the fuel cell, without generating electricity. Heat is also generated when the “crossed over” methanol is oxidized in the cathode chamber. Methanol crossover occurs because present membrane electrolytes are permeable (to some degree) to methanol and water. One method of reducing methanol crossover is to introduce the methanol in an aqueous solution, thus providing the fuel cell with little more methanol than is required for the immediate reaction consumption, minimizing crossover without depriving the fuel cell of the necessary fuel. Methanol is carried over in part by electro-osmotic drag, along with the water in solution, by the essential proton flux of cell operation. Another means to diminish this characteristic is to supply the cell with methanol in vapor form. Details of a technique for providing fuel in such vapor form are set forth in commonly owned U.S. patent application Ser. No. 10/413,986, filed on Apr. 15, 2003, for a VAPOR FEED FUEL CELL WITH CONTROLLABLE FUEL DELIVERY, which is incorporated by reference herein in its entirety.
Many fuel cell systems are run at approximately room temperature, however, in order to obtain an adequate power output and maintain efficiency, such fuel cells are heavily catalyzed, which requires significant amounts of precious metal such as platinum. The cost of platinum has sharply increased in recent years. In order to increase the power output and efficiency of the fuel cell system without increasing the catalyst load, the fuel cell can be run at hotter temperatures in order to produce faster reaction kinetics. For the electrochemical reactions to occur at a higher rate, it is preferable to run the cell at as high a temperature as practical. However, the water needed to keep the membrane hydrated and the water needed for the anodic reaction can vaporize and thus is lost from the reaction zone at such higher temperatures.
Thus, running a fuel cell at higher temperatures requires a novel water balance management scheme or a dilute fuel in order to maintain the essential balance for efficient operation of the fuel cell and to assure that the electrolyte material remains hydrated, to provide both a local aqueous environment for the cell reactions, and a protonically conductive electrolytic medium. On the other hand, excessive hydration must be avoided to prevent flooding of the cathode with liquid water, which could restrict oxygen access to the cathode reaction site.
U.S. Pat. No. 5,432,020 to Fleck, 1995 describes a method for recycling some of the effluent water from the cell reaction to humidify incoming oxygen-supplying air, thus preventing excessive cathode drying by water evaporation to the air. It is also known in the art to apply a water-permeable membrane between incoming and exiting cathode-side flow streams using an external heat exchanger component to recycle reaction-product heat and water from the cell for warming and humidification of incoming reactants (see for example U.S. Pat. Nos. 6,106,964 and 6,416,895 to Voss; and U.S. Pat. No. 6,864,005 to Mossman). However, these prior techniques for recycling effluent water have required undesirably large or complex support devices external to the fuel cell itself, increasing cost, size and depriving the system of useful power density due to parasitic losses. Other approaches to water recovery and recirculation in a fuel cell are limited to liquid water, for use in aqueous-solution fueled cells (see for example U.S. Pat. No. 6,989,206 to Drake which teaches a water-permeable membrane for liquid recovery, or U.S. Pat. No. 6,759,154 to O'Brien, which teaches an air-conditioning system to create and capture liquid condensate for return to incoming reactant flow). These prior means require either management of liquid water or use of add-on equipment external to the fuel cell structure; adding cost, size, and risk of operating failure.
To summarize, there are several different ways which have been suggested in order to balance the water in a fuel cell system. The first is to introduce water from an outside source into the incoming air on the cathode side. Another method is to collect liquid water from the cell reaction within the cathode chamber and to deliver it back into the incoming air inlet. In still other environments, a heat exchanger/condenser, which is a discrete device, has been added on to the cathode side to capture the vapor which is out-going and allow it to travel back over to the inlet side. Other discrete devices in higher power fuel cell systems provide for a separate cooling system that circulates water around the condenser. A part of such cooling water can also be used to return heat and moisture to incoming air, but this requires a separate loop of pipes and conduits to be attached to the fuel cell system. Alternatively, one can operate the fuel cell at a lower temperature and sacrifice power output of the fuel cell, especially in high ambient temperatures.
Such retrofitted components add cost, size and complexity and increases the risk of operating failure of the fuel cell system. This is particularly true in smaller handheld devices in which direct oxidation fuel cells such as direct methanol fuel cells are used.
It is thus an object of the present invention to provide a fuel cell system which includes water balance and allows for higher operating temperatures. It is a further object of the invention to provide a system that includes more humidified air supply to the fuel cell without requiring an additional add-on discrete device in addition to fuel cell system components.